[AArch64] Lowering and legalization of strict FP16

For strict FP16 to work correctly needs some changes in lowering and
legalization:
 * SelectionDAGLegalize::PromoteNode was missing handling for some
   strict fp opcodes.
 * Some of the custom lowering of strict fp operations needed to be
   adjusted to work with FP16.
 * Custom lowering needed to be added for round-to-int operations.

With this, and the previous patches for the rest of the strict fp
isel, we can set IsStrictFPEnabled = true.

Differential Revision: https://reviews.llvm.org/D115620